1. Field of the Invention
The present invention relates to an electromagnetic coil assembly and an electromagnetic actuator which are used in a torque transfer mechanism for an electronically controlled four-wheel drive vehicle.
2. Description of the Related Art
For example, a torque transfer mechanism for a four-wheel drive vehicle includes a pair of left and right planetary gear sets and a pair of brake mechanisms for variably controlling the torque of a sun gear connecting to the respective planetary gear sets. Each brake mechanism includes a wet multi-plate brake and an electromagnetic actuator for actuating the multi-plate brake mechanism.
The electromagnetic actuator includes, in turn, a core (a yoke) having an annular groove, an electromagnetic coil inserted into the annular groove in the core, and armature disposed in such a manner as to face the core with a predetermined gap being provided therebetween, and a piston coupled to the armature.
When a current is applied to the electromagnetic coil, the armature is drawn towards the core by the solenoid to thereby generate a thrust. The piston integrally coupled to the armature presses against the multi-plate brake by virtue of the thrust so generated, whereby a brake torque is generated.
An output torque that is transferred to left and right rear axles can be variably controlled by controlling the value of a current conducted to left and right electromagnetic coils based on a turning direction and a steering force or a steering angle.
An electromagnetic coil is in general use as a conventional electromagnetic coil in which a wire is directly wound around a bobbin formed from a resin material. This electromagnetic coil is inserted into an annular groove in a yoke formed from a magnetic material, and thereafter, a potting material is infected thereover so that the electromagnetic coil is fixed to the yoke.
[Related Art]
    1. JP-A-5-109571    2. JP-A-8-316082    3. JP-A-9-55328    4. JP-A-2001-76927    5. JP-A-2002-57025    6. JP-B-2966162
In the electromagnetic coil in which the wire is wound around the bobbin, a tension load resulting when the wire is wound around the bobbin remains. In addition, when the temperature environment of the electromagnetic coil becomes severe, a stress is generated due to coefficients of linear expansion relative to change in temperature of the yoke formed from the magnetic material, and the resin material constituting the bobbin of the electromagnetic coil and the potting material being different.
There is a problem that the stress attributed to the coefficients combined with the remaining tension results in breakage of the bobbin formed from the resin material or breakage of the wire. In particular, in a case where the electromagnetic actuator has a search coil for detecting the magnetic field of the electromagnetic coil, since the wire of the search coil is thin, the wire is easy to be broken.